City of Light: The Story of Fiber Optics

134 CITY OF LIGHT
Fused silica was a logical starting point. Maurer had recognized its low
scattering back in 1956, and the lab had plenty of samples of the two types
Corning manufactured—pure fused silica and titanium-doped ULE glass. Ideally,
he would have preferred to use the pure material for the core, where
most light traveled, to limit impurity absorption. However, like most impurities
titanium increases the refractive index of silica, so the titanium-doped
silica had to be the core, with pure fused silica as the cladding.
‘‘The choice of silica had a lot of disadvantages to it, which concerned me
considerably,’’ recalls Maurer. He had a long catalog of worries. No one knew
the lower limits of glass absorption, or what glasses would be clearest. It
wasn’t certain if anything could meet Kao’s goal of 20 decibel per kilometer
fibers. ‘‘Everything absorbs light; it’s just a question of what that level of
absorption is,’’ he explains.
Another concern was the high temperature needed to draw silica fibers.
Oxygen can escape from very hot glass, leaving defects called ‘‘color centers’’
because they absorb light. ‘‘Putting the dopant in the core is a lousy idea in
that it gives you great opportunity to generate these color centers,’’ says
Maurer. 11
He knew it was a gamble and thought the odds were against success. However,
Armistead thought the payoff was worth the risk, and Maurer agreed. It
wasn’t a big investment, or even a full-time job for anyone. They wanted to see
if anything in Corning’s considerable bag of glass tricks could beat the odds.
Maurer enlisted the help of veteran Corning glass specialist Frank Zimar,
who had some experience with glass fibers. Zimar also had built a furnace
that could heat glass above 2000�C (3600�F), the only one at Corning—or
any other company working on optical fibers—which was hot enough to
draw fibers from fused silica. He started by machining samples of raw fused
silica, making thin rods of titanium-doped silica and drilling out tubes of pure
silica. Then he inserted the rods into the tubes, essentially recreating the rodin-tube
process that Larry Curtiss had developed a decade earlier. 12
Maurer also put optical fibers on a list of potential projects for graduate
students working during the summer of 1967. He wanted to find a student
who could make single-mode fibers and measure their properties. The project
caught the eye of Cliff Fonstad, an MIT student interested in both electronics
and materials. Fonstad started with Snitzer’s papers on single-mode fibers, but
realized his limited time would constrain his work. He took the simplest approach
he could think of, threading an unclad fiber from Corning’s fiberbundle
plant through a glass capillary tube from the laboratory stockroom,
and handing the assembly to Zimar to draw into a fiber.
They succeeded in making single-mode fibers, but the quality wasn’t very
good. Fonstad was using ordinary glass, and he hadn’t worried about preparing
the glass surfaces. Flaws scattered the red light from a helium-neon
laser out of the fiber and the loss was high. 13 Fonstad went back to MIT to
study other things, but his results encouraged Maurer to think that better
materials and better preparation would yield better results. He convinced Armistead
to invest a bit more time.